Doppler (D) ultrasound imaging has been used to measure velocity of flow of blood cells inside a defined sub-portion of a blood vessel of interest. The Doppler gate (which defines the Doppler sample volume) has been manually placed using B mode imaging and in connection with color flow mapping (CFM), for example, in a B+CFM duplex mode. The CFM shows relative blood flow direction, determined based on a phase shift between returning frequencies and transmitted frequency, with positive shifts indicating blood is moving away from the transducer, and negative shifts indicating blood is moving towards the transducer.
For this, the transducer is positioned so that the B mode image shows the blood vessel of interest, and the CFM is superimposed there over. The user then places the gate along a line of insonation about a cross section of the vessel, using the CFM as a guide.
Velocity is calculated from the Doppler frequency shift according to EQUATION 1:
                    V        =                              C                          2              ⁢                              f                o                            ⁢              cos              ⁢                                                          ⁢              ψ                                ⁢          Δ          ⁢                                          ⁢                      f            .                                              EQUATION        ⁢                                  ⁢        1            where V represents flow velocity, Δf represents the Doppler shift, f0 represents the original transmit frequency, C represents the speed of sound in soft tissue, and ψ represents the angle between the beam and the blood flow. From EQUATION 1, V is inversely proportional to cos ψ, and the calculation of V approaches zero (0) as ψ approaches ninety degrees (90°) (i.e., cos 90=0). An angle of approximately forty-five degrees (45°) to sixty degrees (60°) has been used to obtain velocity estimations.
In order to reduce measurement inaccuracy, the measurement situation is modified via electronic steering and/or manual maneuvering of the transducer elements so that the Doppler angle is approximately forty-five degrees (45°) to sixty degrees (60°).
Angle correction is the process of determining the correct value of ψ. For this, a user-turnable indicator (angle correction) marks the direction of the flow in the color flow mapped image at an approximate center region of the gate. The user eyeballing the vessel in the B-mode image then turns the angle correction indicator until it is aligned with the vessel walls. Having done so, the system calculates the angle between insonation and angle correction indicator. The beam is electronically steered by adjusting the transmit and/or receive profile of the transducer elements or mechanically adjusted until ψ is approximately forty-five degrees (45°) to sixty degrees (60°). The possibilities of electronic steering (Doppler steering angle) are typically limited to three choices, e.g. −20 degrees, 0 degrees, or +20 degrees. Angle correction changes ψ in the EQUATION 1, resulting in a different estimate of the velocity of the blood.
The sensitivity of the Doppler and of the color flow mapping is determined by the pulse repetition frequency (PRF). A sufficiently high PRF is needed to prevent ambiguous velocity estimates (aliasing), but an unnecessarily high PRF will cause the frequency shift for slow flow to be less accurate or even go undetected. Therefore the user will typically adjust the PRF by viewing the color flow map image looking for signs of aliasing.
The output of the Doppler processing is presented not only as maximum or mean velocities but also as a spectrum. The spectrum represents clinical information in a form that a trained user can recognize as an image. Approximately half of the time the spectrum will be presented in a manner that the user can readily understand, the other half of the time the spectrum will be inverted, so that the systolic peaks are pointing downwards. In the latter case, the user will manually invert the spectrum to keep the spectrum above the baseline.
The peak systolic velocity of the internal carotid artery is clinically used as an indicator for the likelihood of a future stroke. To determine the point of highest velocity, the user from the color flow map first tries to identify the approximate point of highest velocity. The highest velocity within a color region of interest is not always evident. Therefore, the user will typically manually move the Doppler gate around in the chosen area to find the highest Doppler shift/velocity.
Volume flow rate has been used for some applications such as dialysis graft assessment. The volume flow rate may be computed using a time average mean velocity of the blood cells passing through the opening of the Doppler gate multiplied with an area of a cross-section of the vessel, as shown in EQUATION 2:
                              Q          =                                    v              TAM                        ⁢                                          π                ⁡                                  (                                      d                    2                                    )                                            2                                      ,                            EQUATION        ⁢                                  ⁢        2            where Q is the volume flow rate, νTAM is the time average mean velocity of the blood cells passing through the gate opening having a speed above the cut-off value determined by the setting of the Doppler Wall filter, and d is the vessel diameter. Outside the vessel, the speed of the scatterers is zero, so νTAM is the same as if the Doppler gate had been adjusted to only cover the vessel. To determine the vessel's cross-section area, the diameter d is used. The diameter is manually determined by the user eyeballing the location of the inner wall of the vessel at the Doppler gate. Then the user manually measures the diameter. Accurate determination of the diameter is particularly important because the volume flow rate is proportional to the square of the diameter.
Prior to the Doppler measurement, the Doppler sample volume size (Doppler gate) must have been set to encompass the entire vessel. If the sample volume size is too small, the average velocity estimate may be too large because the gate then only incorporates the flow in the center of the vessel, and if the sample volume size is too large, the Doppler may pick up signal from more than the one vessel being the target for volume flow rate estimation. Likewise, if angle correction is not employed or not accurate, the velocity estimate may be erroneous (too large or too small), as the denominator of EQUATION 1 increases (or decrease) with decreasing (or increasing) angle. Moreover, manually identification of the vessel diameter is subject to operator error, and may lead to an erroneous volume flow rate.